The coronary sinus is a collection of veins joined together to form a large vessel that collects blood from the myocardium. The coronary sinus runs transversely in the groove between the left atrium and left ventricle on the posterior surface of the heart, and it empties into the right atrium. Leads designed for placement in the coronary sinus can include electrodes that can be positioned in the atrioventricular (AV) groove region between the left atrium and left ventricle or in association with the left ventricle, for example.